The main goal of the project is for the system to be fault-tolerant by detecting and repairing its own faults on the fly, without user intervention. The main uses of the operating system are envisaged to be embedded systems and education.[2]

Reflecting on the nature of monolithic kernel based systems, where a driver (which has, according to MINIX creator Tanenbaum, approximately 3–7 times as many bugs as a usual program)[11] can bring down the whole system,[12] MINIX 3 aims to create an operating system that is a "reliable, self-healing, multiserver UNIX clone".[13]

In order to achieve that, the code running in kernel must be minimal, with the file server, process server, and each device driver running as separate user-mode processes. Each driver is carefully monitored by a part of the system known as the reincarnation server. If a driver fails to respond to pings from the reincarnation server, it is shut down and replaced by a fresh copy of the driver.

In a monolithic system, a bug in a driver can easily crash the whole kernel, something that is much less likely to occur in MINIX 3.[14]

MINIX 3 was publicly announced on 24 October 2005 by Andrew Tanenbaum during his keynote speech on top of the ACM Symposium Operating Systems Principles conference. Although it still serves as an example for the new edition of Tanenbaum and Woodhull's textbook, it is comprehensively redesigned to be "usable as a serious system on resource-limited and embedded computers and for applications requiring high reliability."

Monolithic operating systems such as Linux and FreeBSD and hybrids like Windows have millions of lines of kernel code. In contrast, MINIX 3 has about 6,000 lines of executable kernel code,[17] which can make problems easier to find in the code.

In monolithic kernels, device drivers reside in the kernel itself. This means that when a new peripheral is installed, unknown, untrusted code is inserted in the kernel. A single bad line of code in a driver can bring down the system.

In MINIX 3, each device driver is a separate user-mode process. Drivers cannot execute privileged instructions, change the page tables, perform arbitrary input/output (I/O), or write to absolute memory. They have to make kernel calls for these services and the kernel checks each call for authority.

In monolithic kernels, a driver can write to any word of memory and thus accidentally trash user programs.

In MINIX 3, when a user expects data from, for example, the file system, it builds a descriptor telling who has access and at what addresses. It then passes an index to this descriptor to the file system, which may pass it to a driver. The file system or driver then asks the kernel to write via the descriptor, making it impossible for them to write to addresses outside the buffer.

Dereferencing a bad pointer within a driver will crash the driver process, but will have no effect on the system as a whole. The reincarnation server will restart the crashed driver automatically. For some drivers (e.g., disk and network) recovery is transparent to user processes. For others (e.g., audio and printer), the user may notice. In monolithic kernels, dereferencing a bad pointer in a driver normally leads to a system crash.

If a driver gets into an infinite loop, the scheduler will gradually lower its priority until it becomes idle. Eventually the reincarnation server will see that it is not responding to status requests, so it will kill and restart the looping driver. In a monolithic kernel, a looping driver could hang the system.

MINIX 3 uses fixed-length messages for internal communication, which eliminates certain buffer overflows and buffer management problems. Also, many exploits work by overrunning a buffer to trick the program into returning from a function call using an overwritten stack return address pointing into attacker controlled memory, usually the overrun buffer itself. In MINIX 3, this attack is mitigated because instruction and data space are split and only code in (read-only) instruction space can be executed, commonly known as Data Execution Prevention. However attacks which rely on running legitimately executable memory in a malicious way (return-to-libc, return-oriented programming) are not prevented by this mitigation.

Device drivers obtain kernel services (such as copying data to users' address spaces) by making kernel calls. The MINIX 3 kernel has a bit map for each driver specifying which calls it is authorized to make. In monolithic kernels, every driver can call every kernel function, authorized or not.

The kernel also maintains a table telling which I/O ports each driver may access. As a result, a driver can only touch its own I/O ports. In monolithic kernels, a buggy driver can access I/O ports belonging to another device.

A special process, called the reincarnation server, periodically pings each device driver. If the driver dies or fails to respond correctly to pings, the reincarnation server automatically replaces it with a fresh copy. The detection and replacement of non-functioning drivers is automatic, without any user action required. This feature does not work for disk drivers at present, but in the next release the system will be able to recover even disk drivers, which will be shadowed in random-access memory (RAM). Driver recovery does not affect running processes.

When an interrupt occurs, it is converted at a low level to a notification sent to the appropriate driver. If the driver is waiting for a message, it gets the interrupt immediately; otherwise it gets the notification the next time it does a RECEIVE to get a message. This scheme eliminates nested interrupts and makes driver programming easier.

As can be seen, at the bottom level is the microkernel, which is about 4,000 lines of code (mostly in C, plus a small amount of assembly language). It handles interrupts, scheduling, and message passing. It also supports an API of about 30 kernel calls that authorized servers and drivers can make. User programs cannot make these calls. Instead, they can issue POSIXsystem calls which send messages to the servers. The kernel calls perform functions such as setting interrupts and copying data between address spaces.

At the next level up, there are the device drivers, each one running as a separate user-mode process. Each one controls some I/O device, such as a disk or printer. The drivers do not have access to the I/O port space and cannot issue I/O instructions directly. Instead, they must make kernel calls giving a list of I/O ports to write to and the values to be written. While there is a small amount of overhead in doing this (typically 500 ns), this scheme makes it possible for the kernel to check authorization, so that, for example, the audio driver cannot write on the disk.

At the next level there are the servers. This is where nearly all the operating system functionality is located. User processes obtain file service, for example, by sending messages to the file server to open, close, read, and write files. In turn, the file server gets disk I/O performed by sending messages to the disk driver, which actually controls the disk.

One of the key servers is the reincarnation server. Its job is to poll all the other servers and drivers to check on their health periodically. If a component fails to respond correctly, or exits or gets into an infinite loop, the reincarnation server (which is the parent process of the drivers and servers) kills the faulty component and replaces it with a fresh copy. In this way the system is automatically made self-healing without interfering with running programs.

Currently the reincarnation server, the process server, and the microkernel are part of the trusted computing base. If any of them fail, the system crashes. Nevertheless, reducing the trusted computing base from 3-5 million lines of code found in Linux and Windows systems to about 20,000 lines greatly enhances system reliability[citation needed].

MINIX 3 does the same, and provides a modern operating system with many newer tools and many Unix applications.[18] Prof. Tanenbaum once said:

“

Please be aware that MINIX 3 is not your grandfather's MINIX ... MINIX 1 was written as an educational tool ... MINIX 3 is that plus a start at building a highly reliable, self-healing, bloat-free operating system ... MINIX 1 and MINIX 3 are related in the same way as Windows 3.1 and Windows XP are: same first name.[13]

”

There have also been many improvements in the structure of the kernel since MINIX 2 was released, making the operating system more reliable.[19] MINIX version 3.1.5 was released 5 Nov 2009. It contains X11, Emacs, vi, cc, GCC, Perl, Python, Almquist shell, Bash, Z shell, FTP client, SSH client, Telnet client, Pine, and over 400 other common Unix utility programs. With the addition of X11, this version marks the transition away from a text-only system. Another feature of this version, which will be improved in future ones, is the ability of the system to withstand device driver crashes, and in many cases having them automatically replaced without affecting running processes. In this way, MINIX is self-healing and can be used in applications demanding high reliability.

MINIX 3.2.0 was released in February 2012. This version has many new features, including the Clang compiler, experimental symmetric multiprocessing support, procfs and ext2fs filesystem support, and GDB. Several parts of NetBSD have also been integrated in the release, including the bootloader, libc and various utilities and other libraries.[20]

MINIX 3.3.0 was released in September 2014. This release is the first version to support the ARM architecture in addition to x86. It also supports a NetBSDuserland, with thousands of NetBSD packages running right out of the box.

^Tanenbaum, Andy (2006-09-25). "Introduction to MINIX 3". OSnew. OSnews. Retrieved 2008-07-04. From Rebirth section: "Various studies have shown that software broadly contains something like 6-16 bugs per 1000 lines of code and that device drivers have 3-7 times as many bugs as the rest of the operating system. When combined with the fact that 70% of a typical operating system consists of device drivers, it is clear that device drivers are a big source of trouble. For Windows XP, 85% of the crashes are due to bugs in device drivers. Obviously, to make OSes reliable, something has to be done to deal with buggy device drivers. Building a reliable system despite the inevitable bugs in device drivers was the original driving force behind MINIX 3."